Mono-adsorber PSAs generally comprise a single capacity, simple or of the so-called segregation type, or two capacities, also simple or of the so-called segregation type. Examples are described particularly in U.S. Pat. No. 4,561,865 (Greene & Kellogg), U.S. Pat. No. 4,948,391 (Vacuum Optics Corp.), U.S. Pat. No. 4,892,566 (Air Sep. Corp.), U.S. Pat. No. 5,370,728 (Praxair) , U.S. Pat. No. 5,415,683 (Praxair) , U.S. Pat. No. 5,565,018 (Praxair) , EP 0 663 229 (Sumitomo Seika), EP 0 743 087 (L'Air Liquide). The solutions with a single capacity are preferable as to cost of equipment but do not permit optimization of the cycle nor volumes of gas to be stored. The solutions with capacities of the segregation type permit such optimizations but at the price of very high cost.